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.TH THR_KEYCREATE 3C "Nov 2, 2007"
.SH NAME
thr_keycreate, thr_keycreate_once, thr_setspecific, thr_getspecific \-
thread-specific data functions
.SH SYNOPSIS
.LP
.nf
cc -mt [ \fIflag\fR... ] \fIfile\fR... [ \fIlibrary\fR... ]
#include <thread.h>

\fBint\fR \fBthr_keycreate\fR(\fBthread_key_t *\fR\fIkeyp\fR,
     \fBvoid (*\fR\fIdestructor\fR)(void *));
.fi

.LP
.nf
\fBint\fR \fBthr_keycreate_once\fR(\fBthread_key_t *\fR\fIkeyp\fR,
     \fBvoid (*\fR\fIdestructor\fR)(void *));
.fi

.LP
.nf
\fBint\fR \fBthr_setspecific\fR(\fBthread_key_t\fR \fIkey\fR, \fBvoid *\fR\fIvalue\fR);
.fi

.LP
.nf
\fBint\fR \fBthr_getspecific\fR(\fBthread_key_t\fR \fIkey\fR, \fBvoid **\fR\fIvaluep\fR);
.fi

.SH DESCRIPTION
.SS "Create Key"
.sp
.LP
In general, thread key creation allocates a key that locates data  specific to
each thread in the process. The key is global to all threads in the process,
which allows each thread to bind a value to the key once the key has been
created. The key independently maintains specific values for each binding
thread. The  \fBthr_keycreate()\fR function allocates a global \fIkey\fR
namespace, pointed to by \fIkeyp\fR, that is visible to all threads in the
process. Each thread is initially bound to a private element of this \fIkey\fR,
which allows access to its thread-specific data.
.sp
.LP
Upon key creation, a new key is assigned the value  \fINULL\fR for all active
threads. Additionally, upon thread creation, all previously created keys in the
new thread are assigned the value  \fINULL.\fR
.sp
.LP
Optionally, a destructor function  \fIdestructor\fR can be associated with each
\fIkey\fR. Upon thread exit, if a \fIkey\fR has a non-null \fIdestructor\fR
function and the thread has a non-null \fIvalue\fR associated with that
\fIkey\fR, the \fIdestructor\fR function is called with the current associated
\fIvalue\fR. If more than one \fIdestructor\fR exists for a thread when it
exits, the order of destructor calls is unspecified.
.sp
.LP
An exiting thread runs with all signals blocked. All thread termination
functions, including thread-specific data destructor functions, are called with
all signals blocked.
.sp
.LP
The \fBthr_keycreate_once()\fR function is identical to the
\fBthr_keycreate()\fR function except that the key pointed to by \fIkeyp\fR
must be statically initialized with the value \fBTHR_ONCE_KEY\fR before calling
\fBthr_keycreate_once()\fR and the key will be created exactly once.  This is
equivalent to using \fBpthread_once()\fR to call a onetime initialization
function that calls \fBthr_keycreate()\fR to create the data key.
.SS "Set Value"
.sp
.LP
Once a key has been created, each thread can bind a new \fIvalue\fR to the key
using \fBthr_setspecific()\fR. The values are unique to the binding thread and
are  individually maintained.  These values continue for the life of the
calling thread.
.sp
.LP
Proper synchronization of  \fIkey\fR storage and access must be ensured by the
caller. The \fIvalue\fR argument to \fBthr_setspecific()\fR is generally a
pointer to a block of dynamically allocated memory reserved by the calling
thread for its own use. See \fBEXAMPLES\fR below.
.sp
.LP
At thread exit, the \fIdestructor\fR function, which is associated at time of
creation,   is called and it uses the specific  key value as its sole argument.
.SS "Get Value"
.sp
.LP
\fBthr_getspecific()\fR stores the current value bound to \fIkey\fR for the
calling thread into the location pointed to by \fIvaluep\fR.
.SH RETURN VALUES
.sp
.LP
If successful, \fBthr_keycreate()\fR, \fBthr_keycreate_once()\fR,
\fBthr_setspecific()\fR and \fBthr_getspecific()\fR return 0. Otherwise, an
error number is returned to indicate the error.
.SH ERRORS
.sp
.LP
If the following conditions occur, \fBthr_keycreate()\fR and
\fBthr_keycreate_once()\fR return the corresponding error number:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 10n
The system lacked the necessary resources to create another thread-specific
data key.
.RE

.sp
.ne 2
.na
\fB\fBENOMEM\fR\fR
.ad
.RS 10n
Insufficient memory exists to create the key.
.RE

.sp
.LP
If the following conditions occur, \fBthr_setspecific()\fR returns the
corresponding error number:
.sp
.ne 2
.na
\fB\fBENOMEM\fR\fR
.ad
.RS 10n
Insufficient memory exists to associate the value with the key.
.RE

.sp
.LP
The \fBthr_setspecific()\fR function returns the corresponding error number:
.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fIkey\fR value is invalid.
.RE

.SH EXAMPLES
.LP
\fBExample 1 \fRCall the thread-specific data from more than one thread without
special initialization.
.sp
.LP
In this example, the thread-specific data in this function can be called from
more than one thread without special initialization. For each argument passed
to the executable,  a thread is created and privately bound to the string-value
of that argument.

.sp
.in +2
.nf
/* cc -mt thisfile.c */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <thread.h>

void *thread_specific_data(void *);
void cleanup(void*);
#define MAX_ARGC 20
thread_t tid[MAX_ARGC];
int num_threads;

int
main(int argc, char *argv[]) {
  int i;
  num_threads = argc - 1;
  for (i = 0; i < num_threads; i++)
     thr_create(NULL, 0, thread_specific_data, argv[i+1], 0, &tid[i]);
  for (i = 0; i < num_threads; i++)
     thr_join(tid[i], NULL, NULL);
  return (0);
} /* end main */

void *
thread_specific_data(void *arg) {
  static thread_key_t key = THR_ONCE_KEY;
  char *private_data = arg;
  void *tsd = NULL;
  void *data;

  thr_keycreate_once(&key, cleanup);
  thr_getspecific(key, &tsd);
  if (tsd == NULL) {
       data = malloc(strlen(private_data) + 1);
       strcpy(data, private_data);
       thr_setspecific(key, data);
       thr_getspecific(key, &tsd);
  }
  printf("tsd for %d = %s\en", thr_self(), (char *)tsd);
  thr_getspecific(key, &tsd);
  printf("tsd for %d remains %s\en", thr_self(), (char *)tsd);
  return (NULL);
} /* end thread_specific_data */

void
cleanup(void *v) {
  /* application-specific clean-up function */
  free(v);
}
.fi
.in -2

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Committed
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.sp
.LP
.BR pthread_once (3C),
.BR thr_exit (3C),
.BR attributes (7),
.BR standards (7)
.SH WARNINGS
.sp
.LP
The  \fBthr_getspecific()\fR and \fBthr_setspecific()\fR functions can be
called either explicitly or implicitly from a thread-specific data destructor
function. Calling \fBthr_setspecific()\fR from a destructor can result in lost
storage or infinite loops.
